Dirac and Weyl semimetals have been under intense investigation due to topologically protected Weyl nodes and Fermi arc states connecting the node projections at a surface. Recently, Na3Bi, Cd3As2, PtSe2, and PtTe2 have been experimentally confirmed to be Dirac semimetals (DSMs), where the Dirac nodes are stabilized by crystal rotational symmetries. In the presence of an external magnetic field, the DSMs become Weyl semimetals. In heterostructures involving DSMs, charge transfer may occur at the interfaces, which can be used to probe and control their bulk and surface topological properties through surface-bulk connectivity. Here we demonstrate that despite a band gap in thin DSM films, asymmetric charge transfer at the surface enables one to accurately identify locations of the Dirac-node projections from gapless band crossings and to examine and engineer properties of the topological Fermi-arc surface states connecting the projections, by simulating adatom-adsorbed DSM films using a first-principles method and by comparing with the effective model. We show that as the amount of charge transfer varies, unique spin textures near the projections and a separation between the Fermi-arc states change. Our results can be observed by top or bottom gating without adatoms.